1. Field of the Invention
This invention relates in general to fiber optic transmission lines. More specifically the invention is directed to a fiber optic transmission line having a single crystal core having a non-linear optical characteristic and a glass cladding surrounding the core.
2. Description of Related Art
One of the most important applications for semiconductor lasers is for use in digital optical communications systems. The development of the laser and the optical fiber has brought about a revolution in communication system design. The optical fiber is a dielectric waveguide. A light wave transmitted into the fiber at one end can propagate through the fiber with very low attenuation making possible transmission over very long distances without amplification.
Attenuation of laser light is a limiting factor in optical fiber communications. There is always a maximum transmitter power level and a minimum useful received power level. The difference between these two represents transmission loss due primarily to fiber attenuation.
When light, such as from a laser, is incident upon any discontinuity in a communication medium, such as an optical fiber, some energy will be reflected from the discontinuity. In some cases the discontinuity is due to random irregularities or imperfections in an otherwise uniform medium. By eliminating such deformations in an optical medium, losses due to attenuation and reflection are minimized significantly.
Conventional fiber optic transmission lines comprise a core surrounded by a glass cladding. The propagation of a beam of light in the core of an optical fiber generates non-linear optical effects, such as second-harmonic generation, proportionate to the intensity of the beam. The non-linear optical phenomena become appreciable for light beams of higher intensity. Functional non-linear optical waveguide devices are essentially designed and fabricated based on their response to a specific intensity light beam.
The phenomena of second-harmonic generation is critical in the domain of non-linear optics because a incident wave characteristically will undergo wavelength conversion. A fiber optic transmission line must be designed that is best suited (i.e., minimize losses) for propagation of light beams.
Second harmonic generators designed to read and write to/from an optical medium such as an optical disk are well known. Optic fiber transmission lines having a single crystal core surrounded by a glass cladding are commonly used as a communication medium to/from such optical devices. The core generally used to propagate high intensity light is constructed of a single crystal dielectric material. Any material may be suitable for wavelength conversion if signal (laser light) losses are minimized. Moreover, wavelength conversion efficiency can be made high by making the refractive index of the cladding lower than that of the core by a relatively small ratio resulting in an incident laser light that is propagated in a single mode.
The following is an example of single mode non-linear optical transmission. An optical fiber has a core made of a single crystal of a non-linear optical material and having a refractive index of 1.78. The clad is made of SF11 glass or SFS3 glass having a refractive index of 1.76. Laser light having a wavelength of 0.82 microns is transmitted into the fiber. The light propagates in a single mode and second-harmonic wave is generated with a calculated conversion efficiency of 1% from incident laser light of 1 W, at a fiber length of 10 mm. The SF11 and SFS3 glass cladding contain predetermined quantities of lead oxide (PbO) in order to improve the refractive index that will result in a suitable single mode fiber using the non-linear optical material as a core.
Conventional manufacturing techniques require first forming a cylindrical glass mother material. Several kinds of metal oxide powder of raw materials adhere to the glass mother material during formation and it becomes necessary to wash the glass mother material with an acid solution to remove the surface powders and impurities. However, metal mono-oxide (RO), where R represents a metal atom and O represents an oxygen atom, such as lead oxide (PbO) or the like, easily elute into water or into an acid. The glass mother material that will be used to form the cladding will not be of sufficient water resistance or acid resistance and will result in elusion of metal oxides on the surface of the glass mother material.
Accordingly, when a glass mother material is worked into the shape of a capillary tube so as to form a cladding, uneven portions are formed on the inner wall of the capillary tube attributed to the lead oxide elutions on the glass mother material. These elusion defects are referred to as stain or dimming.
When a single crystal core is made to grow in the capillary tube, dislocation (a defect related to cracking) and peeling from a cladding may be generated in the crystal under influence of the preexisting uneven portions of the capillary tube, so that the propagation losses become high. The problem occurs frequently resulting in a high rate of defective output during formation of fiber optic transmission lines. Furthermore, a low refractive index layer attributed to the eluted metal oxide molecules is present on the inner wall of the capillary tubes. Refractive indices of this layers are different between production.
Therefore, stable manufacturing of a single mode fiber becomes difficult.